[lttng-dev] Using LTTng for existing Qt-based tracepoints?

[Milian Wolff]

On Donnerstag, 19. Oktober 2017 12:00:01 CEST Arnaud Clère wrote:
> Hi all,
> I am evaluating the feasibility of using LTTng as a backend for existing Qt
> tracepoints following the discussions at Qt Contributors Summit :
> https://wiki.qt.io/QtCS2017_Discuss_Qt_Logging_enhancements . The idea
> would be to get less performance impact when tracing and more structured
> data to analyse without changing Qt users' code.
> 
> I see that instead of describing all tracepoints, we could use a generic LTT
> tracepoint like in Python:
> http://git.lttng.org/?p=lttng-ust.git;a=blob;f=liblttng-ust-python-agent/lt
> tng_ust_python.h
> 
> Except, I would like to structure the tracepoint data at run-time following
> a JSON-based data model. I think I could encode that part of CTF
> dynamically by appending binary user data, CTF variants, dynamically-sized
> CTF structures and sequences in a local buffer.
> 
> But I am wondering how to pass this buffer along with its size to LTTng.
> I am afraid a TP_ARGS(void*, buffer) and TP_FIELDS(buffer) would not work
> since it would require LTTng to read the CTF in "buffer" to discover where
> it stops...
> 
> Or should I use some other API?

Hey Arnaud,

Why do we want to use the generic LTTng tracepoint instead of the concrete 
ones? See also the existing work that went into https://codereview.qt-project.org/#/c/185287/ and the dependent changesets. We hope to be able to 
revive this patchset soon.

Cheers

-- 
Milian Wolff | milian.wolff at kdab.com | Senior Software Engineer
KDAB (Deutschland) GmbH&Co KG, a KDAB Group company
Tel: +49-30-521325470
KDAB - The Qt Experts